1) Field of the Invention
The present invention relates to an acousto-optic device suitably used in an optical communication system.
2) Description of the Related Art
An acousto-optic device is a device utilizing an acousto-optic effect that is an interaction between an ultrasonic wave and light, such as an Acousto-Optic Tunable Filter (AOTF). This AOTF is expected to be used for an optical cross connect, an optical switch, light modulation, or the like in addition to an Optical Add-Drop Multiplexer (OADM) in a Wavelength Division Multiplexing (WDM) optical communication system.
An acousto-optic wavelength tunable filter is a wavelength filter using a TE-TM mode conversion caused by an acousto-optic effect that is an interaction between light in an optical waveguide made on a piezoelectric material such as a lithium niobate (LiNbO3) substrate (hereinafter simply referred to LN substrate in some cases), and a Surface Acoustic Wave (SAW) applied onto it.
FIG. 17 is a schematic top view showing a configuration example of a typical AOTF. In this AOTF 100, on an LN substrate 101, a mode conversion unit 103 as an interaction region is formed which comprises of two optical waveguides 103a-1, 103a-2 and a surface acoustic wave guide 103b that is a SAW waveguide made of a thin film formed on the optical waveguides. Polarization beam splitters 102, 104 are formed before and after the mode conversion unit 103 respectively. In addition, a pair of comb electrodes (IDT: Inter Digit Transducer) 105 is formed capable of generating a SAW, which is propagated in the surface acoustic wave guide 103b, when an ultrasonic wave signal [e.g., radio frequency (RF) signal] is applied to it.
In the AOTF 100 configured like this, light inputted is split into TE light and TM light by a polarization beam splitter 102 located at the input side, and then a SAW derived from the comb electrode 105 makes TE/ME conversion to the light having a specific wavelength independently at the optical waveguides 103a-1, 103a-2, and the polarization beam splitter 104 located at the output side performs wave combination. Thus, a device characteristic having no polarization dependence is realized.
Furthermore, in the AOTF 100 described above, there is a correspondence between an optical wavelength for which TE/TM mode conversion is performed and the frequency of a SAW, so that an optical wavelength for which TE/TM mode conversion is performed can be tuned by changing the frequency of the SAW. Specifically, changing an RF signal applied to the IDT 105 for SAW excitation changes the frequency of the SAW excited, and thereby an optical wavelength to which TE/TM mode conversion is performed can be tuned.
Another configuration example of an AOTF is a directional coupling type AOTF 100A as shown in FIG. 18. In this AOTF 100A, an IDT 105A is provided at the left side of the direction of light propagation on the upper reaches of the optical waveguides 103a-1, 103a-2, and two parallel surface acoustic wave guides 103b-1, 103b-2 are provided, in order that a SAW generated by applying an RF signal to the IDT 105A is directionally coupled from the surface acoustic wave guide 103b-1 to the surface acoustic wave guide 103b-2 formed on the optical waveguides 103a-1, 103a-2. In other words, the area of the surface acoustic wave guide 103b-2 formed on the optical waveguides 103a-1, 103a-2 is defined as an interaction area 103A. The same notations in FIG. 18 as ones in FIG. 17 represent substantially similar portions.
Moreover, publicly known arts related to the present invention include ones described in the patent documents 1 to 3 listed below.
In the patent document 1, an acousto-optic tunable filter is described in which the center of the converter causing an acoustic wave beam in response to an electric signal is positioned at one side of the optical axis, and the axis of the acoustic wave beam crosses the optical axis at an oblique angle, in order to reduce side lobes without adding any process stage unnecessary for the manufacture.
In the patent document 2, an optical wavelength variable filter is described which is provided with a distortion adding unit in order to obtain a filter characteristic having a little asymmetry about the central wavelength.
In the patent document 3, an acousto-optic type variable wavelength TE/TM mode converter capable of simultaneously selecting light having multiple wavelengths adjacent to each other, while avoiding the increase of power of a high frequency electric signal.
[Patent Document 1] Japanese Patent Laid-Open (Kokai) NO. HEI 10-10481
[Patent Document 2] Japanese Patent Laid-Open (Kokai) NO. HEI 11-326855
[Patent Document 3] Japanese Patent Laid-Open (Kokai) NO. 2001-209019
However, an acousto-optic device like the AOTF 100 shown in FIG. 17 described above has a problem that because of a limitation on a band which can be generated as a SAW, when it is used as an AOTF, there is a limitation on the range of tunable optical wavelengths to which mode conversion is performed, so that it is difficult to extend the range of tunable optical wavelengths.
That is, in the AOTF 100 shown in FIG. 17, there are adequate frequency ranges in the domain of frequencies applied to the IDT 105 to generate a SAW and in the domain of frequencies capable of propagating in the surface acoustic wave guide 103b, respectively, so that the range of tunable optical wavelengths about optical wavelengths to which mode conversion is performed by the AOTF 100 described above is also decided in connection with the adequate frequency ranges of the IDT 105 and surface acoustic wave guide 103b. 
FIG. 19 shows an example of the relationship between the frequency (horizontal axis) of an electric signal having a constant amplitude applied to the IDT 105 to generate a SAW and the intensity (vertical axis) of a SAW excited correspondingly. For generating a SAW having a specified intensity in a range below a value PM specified as the upper limit of power consumption as shown in FIG. 19, the frequency of the electric signal is limited to the band B1 corresponding to FIG. 19 described above as shown in FIG. 20.
That is, the efficiency of exciting a SAW of the IDT 105 has a large dependence on the frequency of an electric signal applied to the IDT 105. It is known that the frequency of an electric signal supplied to the IDT 105 is reflected in the frequency characteristic of a SAW excited.
In other words, when the power consumption of the power source, which is not shown, for supplying an electric signal to the IDT 105, and the SAW intensity necessary for mode conversion are taken into consideration, the frequency band of RF signals which can be supplied to the IDT 105 is limited to the band B1 shown in FIGS. 19 and 20, and the frequency band of SAWs is also limited to a band corresponding to the band B1.
Furthermore, in the AOTF 100A shown in FIG. 18 described above, the IDT 105A usually excites a SAW of zero order mode, but a SAW excited by the IDT 105A has a limitation on the range of wavelengths which couple to the surface acoustic wave guide 103b-2 as shown in FIG. 21 for example. Because of this, a directional coupling type acousto-optic device also has a limitation on the frequency range of SAWs based on the range of frequencies coupling completely.
In this connection, point C in FIG. 21 represents a case that a SAW of zero order mode excited by the IDT 105A has a wavelength that couples to the surface acoustic wave guide 103b-2 at the downstream end of it. That is, point C represents that a SAW excited by the IDT 105A is required to have a length which is two times as long as that of the surface acoustic wave guides 103b-1, 103b-2 in order to return (completely couple) onto the original axis of the SAW 103b-1.
Furthermore, point D represents a case that a SAW of zero order mode excited by the IDT 105A has a wavelength that couples to the surface acoustic wave guide 103b-2 at the intermediate portion of it. That is, point D represents that the SAW is required to have the same length as that of the surface acoustic wave guides 103b-1, 103b-2 for complete coupling. Thus, as shown in FIG. 21, only SAWs, of SAWs of zero order mode excited by the IDT 105A, having wavelengths (or frequencies) between a SAW wavelength corresponding to point C and a SAW wavelength (or SAW frequency) corresponding to point D can be used. This limitation on SAW wavelength is directly linked to the limitations of optical wavelengths to which mode conversion is possible.
It is therefore desirable to reduce the limitations of wavelengths of SAWs to be generated in order to extend the range of optical wavelengths to which mode conversion is possible.
In the patent documents 1 to 3 described above, any technique for extending the band of wavelengths of SAWs as described above is not described. In the technique described in the patent document 3, adjacent frequencies are applied to different comb electrodes, but the band B1 of RF frequencies are as shown in FIG. 19 described above, that is, the band of SAW frequencies, can not be extended.